Free-standing MOF-derived hybrid porous carbon nanofiber mats

ABSTRACT

According to the present disclosure, a method of fabricating a metal-carbon fibrous structure is provided. The method comprises the steps of: (a) forming a fibrous support structure comprising composite nanocrystals and polymeric fibers, wherein each of the composite nanocrystals comprises metal ions connected by organic ligands; (b) growing the composite nanocrystals on the fibrous support structure; and (c) subjecting the fibrous support structure of step (b) to carbonization to form the metal-carbon fibrous structure, wherein the metal-carbon fibrous structure comprises metal nanoparticles derived from the composite nanocrystals. A metal-carbon fibrous structure comprising carbon based fibers arranged to form a porous network and the carbon based fibers are doped with metal nanoparticles, wherein the carbon based fibers have surfaces which comprise graphitic carbon, is also disclosed herein.

TECHNICAL FIELD

The present disclosure relates to a method of fabricating a metal-carbonfibrous structure and the metal-carbon fibrous structure.

BACKGROUND

Nanoporous carbon materials (NPC) with a high specific surface area,narrow pore-size distribution, good thermal and chemical resistance havebeen considered promising materials for a wide range of applicationssuch as adsorption, separation, sensing, energy storage and conversionetc. To date, efforts have been made in the preparation of NPC materialswith various pore structures using “hard” or “soft” templating methods.These methods, however, tend to suffer from either tedious and costlyfabrication process or insufficient thermal stability that may causedecomposition before the carbonization process completes.

Besides construction of porous structure, it may be more interesting toexplore the possibility of enhancing and/or extending their propertiesby incorporating well-dispersed functional nanoparticles in the NPCmatrix through either a direct-synthesis approach or a post-loadingapproach. However, in many instances, the nanoparticles tend to berandomly dispersed and/or severely aggregated to form undesirable largeparticles. Thus, it becomes challenging to introduce uniformly dispersedcrystalline nanoparticles with a high metal content into the nanoporouscarbon matrix.

Recently, thermal decomposition of metal-organic frameworks (MOFs) orporous coordination polymers (PCPs) has been demonstrated as a facileroute to synthesize hybrid NPCs. MOFs may be microporous materialssynthesized by assembling metal ions with organic ligands in appropriatesolvents. The crystalline structures of MOFs may provide large internalsurface area, extremely high porosity, tunable porosity, and tailoredchemical properties. Utilizing MOFs with various functional metalspecies may provide a great opportunity to develop new types ofcarbon-based nanoporous composites with promising applications. Forexample, via a one-step carbonization of zeolitic imidazolateframework-67 (ZIF-67) crystals, research groups developed nanoporouscarbon particles with well dispersed magnetic cobalt, which exhibited anexcellent adsorption performance towards organic dye in pollutant waterand could be separated by applying an external magnetic field. Otherresearchers reported that synthesized Co-doped zinc oxide-carboncore-shell composite nanoparticles via carbonization of Co-MOF-5crystals demonstrated their suitability as high performance anodematerial for lithium ion batteries with a reversible capacity of 725 mAh g⁻¹ up to the 50^(th) cycle at a current density of 100 mA g⁻¹.Moreover, Al-PCP-derived highly porous carbon had been examined as asensing material for toxic aromatic compounds. The fast response andhigh uptake towards aromatic compounds over the Al-PCP-derived carbonwas about 4 times higher than commercial active carbon. This may be dueto the presence of graphitic sp²-hybridized carbons. The MOF-derivednanoporous carbons may show excellent properties in adsorption,electrochemical capacitance, sensing, and catalysis due to the uniqueadvantages of (1) high surface area and large pore volume, (2) orderedporous structures and narrow pore size distributions without templatingmethods, (3) high content and uniform dispersion of metal nanoparticles,(4) well developed graphitic carbon derived from the ordered crystalstructure, (5) tunable compositions and sizes, and (6) scalablefabrication process.

In previous works, MOF derived carbon materials tend to be fabricated asbulk porous particles via direct carbonization of MOF crystals, anddifficulties in handling may arise for certain applications with theoverall performance and structural integrity adversely affected. Thereis thus a challenge to control the dimensions of MOF crystals and tofabricate free-standing, interconnected carbon architectures derivedfrom MOF assemblies.

For applications as electrode materials in energy storage devices, thereis a need to provide for free-standing carbon membranes which may beused directly without adding any conductive agent and binder. Thisadvantageously simplifies the fabrication process. Furthermore, comparedto separate carbon particles, the interconnected carbon matrix mayprovide shorter and more continuous pathways for electron and masstransportation, and thereby enhancing the electrochemical performance ofthe electrode. Accordingly, there is a need to provide for a method thatleads to such advantages and ameliorates one or more of the limitationsas mentioned above.

There is also a need to provide for a fibrous structure incorporatedwith metal nanoparticles, which not only demonstrates the above benefitsbut can also ameliorate one or more of the above limitations asmentioned above.

SUMMARY

In one aspect, there is a method of fabricating a metal-carbon fibrousstructure comprising the steps of: (a) forming a fibrous supportstructure comprising composite nanocrystals and polymeric fibers,wherein each of the composite nanocrystals comprises metal ionsconnected by organic ligands; (b) growing the composite nanocrystals onthe fibrous support structure; and (c) subjecting the fibrous supportstructure of step (b) to carbonization to form the metal-carbon fibrousstructure, wherein the metal-carbon fibrous structure comprises metalnanoparticles derived from the composite nanocrystals.

In another aspect, there is a metal-carbon fibrous structure comprisingcarbon based fibers arranged to form a porous network and the carbonbased fibers are doped with metal nanoparticles, wherein the carbonbased fibers have surfaces which comprise graphitic carbon.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to like partsthroughout the different views. The drawings are not necessarily toscale, emphasis instead generally being placed upon illustrating theprinciples of the invention. In the following description, variousembodiments of the present disclosure are described with reference tothe following drawings, in which:

FIG. 1 a to FIG. 1 g show a schematic diagram depicting the fabricationprocess of freestanding MOF derived carbon fibrous mats.

FIG. 1 a shows an electrospinning setup.

FIG. 1 b shows a MOF embedded electrospun fibrous mat.

FIG. 1 c shows fibers (in the form of a fibrous mat) soaked in asolution of metal salt precursor (left picture) followed by the slowaddition of organic linker solution via a syringe (right picture).

FIG. 1 d shows a step of secondary growth of MOF crystals on the fibersunder a mild condition of 60° C. within a short duration of 1 hour to 3hours.

FIG. 1 e shows one of the repeated steps of washing the fibrous mat ofFIG. 1 d.

FIG. 1 f shows a retrieved MOF fibrous mat of FIG. 1 e.

FIG. 1 g shows a dried and carbonized MOF based carbon fibrous mat.

FIG. 2 a shows a scanning electron microscopy (SEM) image of zeoliticimidazolate framework-67 (ZIF-67) nanocrystals. Magnification is set at×40000 and the scale bar represents 100 nm. The ZIF-67 nanocrystals havea size of about 200 nm.

FIG. 2 b shows a SEM image of ZIF-67/PAN electrospun fibers.Magnification is set at ×5000 and the scale bar represents 1 μm. FIG. 2b shows the MOF nanocrystals embedded on the fibers. The MOFnanocrystals are located inside the fibers and are also partiallyexposed on the surface of fibers.

FIG. 3 a shows a SEM image of seed-mediated growth of ZIF-67nanocrystals on ZIF-67/PAN electrospun fibers. Magnification is set at×8000 and the scale bar represents 1 μm. The ZIF-67 nanocrystals have asize in the range of 100 nm to 200 nm.

FIG. 3 b shows a SEM image of seed-mediated growth of ZIF-67nanocrystals on ZIF-67/PAN electrospun fibers. Magnification is set at×11000 and the scale bar represents 1 μm. The ZIF-67 nanocrystals have asize in the range of 100 nm to 200 nm.

FIG. 3 c shows a SEM image of seed-mediated growth of ZIF-67nanocrystals on ZIF-67/PAN electrospun fibers. Magnification is set at×50000 and the scale bar represents 100 nm. The ZIF-67 nanocrystals havea size in the range of 100 nm to 200 nm.

FIG. 4 a shows a X-ray powder diffraction (XRD) spectra of pure ZIF-67material.

FIG. 4 b shows a XRD spectra of ZIF-67 nanocrystals.

FIG. 4 c shows a XRD spectra of as-spun ZIF-67/PAN fibers according toembodiments as disclosed herein.

FIG. 4 d shows a XRD spectra of ZIF-67/PAN fibers after secondary growthaccording to embodiments as disclosed herein.

FIG. 5 a shows a SEM image of ZIF-67 derived electrospun carbon (i.e.carbonized) nanofibers. Magnification is set at ×15000 and the scale barrepresents 1 μm.

FIG. 5 b shows a SEM image of ZIF-67 derived electrospun carbonnanofibers. Magnification is set at ×43000 and the scale bar represents100 nm.

FIG. 6 a shows a transmission electron microscopy (TEM) image of ZIF-67derived carbon (i.e. carbonized) nanofibers. The scale bar represents 1μm. From FIG. 6 a , it can be seen that the nanofibers are composed ofinterconnected MOF derived hollow carbon nanocages as building blocks inone dimension (1D).

FIG. 6 b shows a TEM image of ZIF-67 derived carbon nanofibers. Thescale bar represents 1 μm. From FIG. 6 b , it can also be seen that thenanofibers are composed of interconnected MOF derived hollow carbonnanocages as building blocks in one dimension (1D).

FIG. 7 a shows a high resolution TEM image of ZIF-67 derived carbonnanofibers. The scale bar represents 0.2 μm. FIG. 7 a also shows how thefiber surfaces are doped with metal nanoparticles as represented by thedarker shades.

FIG. 7 b shows a high resolution TEM image of ZIF-67 derived carbonnanofibers. The scale bar represents 50 nm. FIG. 7 b also shows how thefiber surfaces are doped with metal nanoparticles as represented by thedarker shades.

FIG. 7 c shows a high resolution TEM image of ZIF-67 derived carbonnanofibers. The scale bar represents 5 nm. FIG. 7 c shows the boundarywhere the graphitic carbon is located.

FIG. 8 shows a Raman spectrum of ZIF-67 derived carbon nanofibers. Theleft and right peaks are the D (amorphous carbon) and G (graphiticcarbon) bands, respectively.

FIG. 9 shows the XRD patterns of Co/C (upper pattern) and Co₃O₄/C (lowerpattern) fibers derived from ZIF-67.

FIG. 10 shows the nitrogen adsorption/desorption isotherms of ZIF-67derived carbon fibers.

FIG. 11 shows a Barrett-Joyer-Halenda (BJH) pore size distribution ofthe ZIF-67 derived carbon fibers.

FIG. 12 shows the initial discharge/charge curves of ZIF-67 derivedCo₃O₄/carbon fibers at a current rate of 50 mA g⁻¹ up to the 10^(th)cycle.

FIG. 13 shows a plot of specific capacity and coulombic efficiency ofthe first 10 cycles of ZIF-67 derived Co₃O₄/carbon fibers at a currentrate of 50 mA g⁻¹.

FIG. 14 shows a rate capacity at different current densities for theZIF-67 derived Co₃O₄/carbon fibers.

FIG. 15 shows a plot of the cyclic stability evaluated at a currentdensity of 100 mA g⁻¹ for 50 cycles based on ZIF-67 derived Co₃O₄/carbonfibers.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific details and embodiments inwhich the invention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and changes may be madewithout departing from the scope of the invention. The variousembodiments are not necessarily mutually exclusive, as some embodimentscan be combined with one or more other embodiments to form newembodiments.

As mentioned above, there is a need to provide for a method and afibrous structure incorporated with metal nanoparticles, both of whichnot only ameliorate one or more of the above drawbacks but also possessthe desirable traits as disclosed above. With this in mind, theinventors have come to discover that electrospinning can be used to makecarbonized fibrous structure incorporated with metal nanoparticleshaving the abovementioned advantages. The method of generatingcarbonized fibrous structure via electrospinning as disclosed hereinalso provides more flexible in its application and processing comparedto bulk synthesis of carbon particles.

Electrospinning may be a straightforward and versatile technique forgenerating continuous nanofibers from solutions of polymers or polymersblends. This approach may be capable of producing nanofibers with mostmaterials (e.g. organic, inorganic or hybrid organic-inorganic), therebyaffording various fibers with the desired composition and surfaceproperties. In the present disclosure, it is discovered thatfree-standing MOF membranes using electrospun nanofibrous mats asskeletons can be developed. This can be used for gas separation anddetection of explosives. This demonstrates the potential of suchnonwoven fiber mats as a new type of porous support in MOF research andapplications.

Accordingly, the present disclosure provides a scalable method tofabricate MOF derived free-standing hybrid porous carbon fibrous matsvia electrospinning, seed-mediated secondary growth of MOF nanocrystalsfollowed by carbonization. Subsequently, post heat treatment may beapplied. The advantageous properties of the resultant nanoporous carbonmats may be demonstrated by application as anode materials for lithiumion batteries.

With the above in mind, embodiments described in the context of thepresent method are analogously valid for the metal-carbon fibrousstructure, and vice versa.

Features that are described in the context of an embodiment maycorrespondingly be applicable to the same or similar features in theother embodiments. Features that are described in the context of anembodiment may correspondingly be applicable to the other embodiments,even if not explicitly described in these other embodiments.Furthermore, additions and/or combinations and/or alternatives asdescribed for a feature in the context of an embodiment maycorrespondingly be applicable to the same or similar feature in theother embodiments.

In the context of the present disclosure, the term “organic” refers tocarbon based materials. For example, the phrase “organic solvent” or“organic solution” refers to a liquid that is carbon based. The organicsolvent or solution may be polar or non-polar.

In the context of the present disclosure, the term “nanoparticle” refersto a particle having one or more dimensions in the range of 1 nm to 100nm. Meanwhile, the term “nanocrystal” refers to a crystalline particlewith at least one dimension measuring less than 1000 nm.

In the context of the present disclosure, the term “nanoporous”, orvariants such as “nanopore”, refers to materials comprising a regularorganic or inorganic framework with a porous structure comprising poresthat are 100 nm or smaller.

In the context of the present disclosure, the term “micropore”, orvariants such as “microporous”, may also be used to refer to materialswith pore sizes of less than 2 nm. In other words, in the context of thepresent disclosure, a nanopore may be specifically referred to as amicropore if the nanopore is less than 2 nm. Meanwhile, the term“mesoporous”, or variants such as “mesopore”, refers to materials withpore sizes from 2 to 50 nm.

The word “substantially” does not exclude “completely” e.g. acomposition which is “substantially free” from Y may be completely freefrom Y. Where necessary, the word “substantially” may be omitted fromthe definition of the invention.

In the context of various embodiments, the articles “a”, “an” and “the”as used with regard to a feature or element include a reference to oneor more of the features or elements.

In the context of various embodiments, the term “about” or“approximately” as applied to a numeric value encompasses the exactvalue and a reasonable variance.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items.

As used herein, the phrase of the form of “at least one of A or B” mayinclude A or B or both A and B. Correspondingly, the phrase of the formof “at least one of A or B or C”, or including further listed items, mayinclude any and all combinations of one or more of the associated listeditems.

Unless specified otherwise, the terms “comprising” and “comprise”, andgrammatical variants thereof, are intended to represent “open” or“inclusive” language such that they include recited elements but alsopermit inclusion of additional, unrecited elements.

The present disclosure relates to a method of fabricating a metal-carbonfibrous structure. The method may comprise a step of forming a fibroussupport structure comprising composite nanocrystals and polymericfibers, wherein each of the composite nanocrystals comprises metal ionsconnected by organic ligands, a step of growing the compositenanocrystals on the fibrous support structure, and a step of subjectingthe fibrous support structure of the preceding step to carbonization toform the metal-carbon fibrous structure, wherein the metal-carbonfibrous structure comprises metal nanoparticles derived from thecomposite nanocrystals.

The method may be carried out in the sequence as described above. Thatis to say, the present method may start with the step of forming, thenthe step of growing and subsequently the step of subjecting the fibroussupport structure to carbonization.

In the context of the present disclosure, the expression “metal-carbon”refers to structures made of carbon based materials incorporating ametal component. The metal components may be in the form of metalnanocrystals, nanoparticles or metal based particles. The metalcomponents may also be in the form of metal oxide based nanoparticles.Meanwhile, the expression “metal-carbon fibrous structure” is meant torefer to the resultant metal-carbon fibrous structure derived from themethod described in the present disclosure.

In the present method, the forming step may be carried out by dispersingthe composite nanocrystals in a polymer solution to form a suspension,and electrospinning the suspension to form the fibrous support structurecomprising the composite nanocrystals and polymeric fibers.

In the present method, the composite nanocrystals may be formed byseparately dissolving a metal nitrate (e.g. metal nitrate hexahydrate)or a metal chloride and the organic ligands in a solvent to form twosolutions, mixing the two solutions to form a precipitate of thecomposite nanocrystals, and collecting and drying the precipitate tothereby form the composite nanocrystals. In other words, the synthesisof the composite nanocrystals may involve the preparation of onesolution comprising the metal nitrate or metal chloride and anotherseparate solution comprising the organic ligands. The earlier metalnitrate or metal chloride solution may be prepared by dissolving themetal nitrate or metal chloride in a solvent and the latter organicligands solution may be prepared by dissolving the organic ligands in aseparate solvent. The dissolution of the organic ligands in the solventmay be carried out under sonication.

The two distinct solutions may then be mixed and stirred for 30 minutesto 1.5 hours, or 1 hour, at room temperature. The room temperature maybe about 20° C. to 30° C. A precipitate of the composite nanocrystalsmay subsequently form during and/or after mixing.

For formation of the composite nanocrystals, the metal nitrate maycomprise any suitable forms of metal nitrate with or without crystal(s)of water. The metal nitrate may be a metal nitrate hexahydrate. Themetal nitrate may comprise or consist of cobalt nitrate, iron nitrate orzinc nitrate. For example, the metal nitrate may comprise or consist ofcobalt nitrate hexahydrate, iron nitrate hexahydrate or zinc nitratehexahydrate. Meanwhile, the metal chloride may comprise or consist ofcobalt chloride, iron chloride or zinc chloride. Accordingly, the metalcomponent of the materials used herein or resulting products may take onvarious valencies or just one form of valency. To illustrate this point,the iron in iron chloride may be iron (II) and/or iron (III), and suchvariations may apply to the other metal components listed herein.

Independently, the organic ligands may comprise or consist of imidazolebased ligands. In various embodiments, the imidazole based ligands maycomprise or consist of imidazolate ligands. In various embodiments, theimidazolate ligands may be 2-methylimidazole (Hmim). Meanwhile, thesolvent for preparing the two solutions as mentioned above may be anaqueous solvent. The solvent may comprise or consist of water and/ormethanol. In various embodiments, the aqueous solvent may be water. Thewater may be deionized or distilled water.

The choice of solvent may modify the evolution of the reaction, alteringthe crystallization rates and nanocrystal sizes. The hydrogen bonddonating ability of the solvent tends to be the main factor that governsthese effects. Using aqueous solvent may accelerate crystallization ofthe ZIF-67 MOF nanocrystals.

Ultimately, to obtain the composite nanocrystals, the precipitate may becollected and dried in vacuum for 12 hours to 24 hours at 40° C. to 100°C. For example, the precipitate may be collected and dried for 24 hoursat 80° C. The collection may be carried out by centrifuging and thenwashing with water, followed by washing with an alcohol (e.g. methanol)for 3 times. This washing procedure may be carried out to wash offunreacted reactants or precursors to prevent further growth orcontamination. The usage of methanol helps with easy removal of excessligands.

The precipitate before or after collection may be a purple colouredprecipitate. The colour of the precipitate may depend on the metalcomponent of the starting materials used to form the compositenanocrystals.

In the context of the present disclosure, the composite nanocrystals maybe referred to as metal-organic frameworks (MOFs), MOF crystals or MOFnanocrystals. MOFs may be compounds comprising metal ions or clusters ofmetal ions coordinated or connected to organic ligands by coordinatebond(s). The metal ions or clusters of metal ions may also be connectedby the organic ligands via coordinate bond(s). MOFs may comprise voidsand hence may be porous. Hence, MOFs or the composite nanocrystals asreferred to in the present disclosure may comprise two main components,which are the metal ions or clusters of metal ions and an organic unit.The organic units may comprise or consist of mono-, di-, tri-, ortetravalent organic ligands. The choice of metal and the ligand maydictate the structure and hence properties of the MOF. For example, themetal's coordination preference may influence the size and shape ofpores by dictating how many ligands can bind to the metal and in whichorientation. Hence, the properties of MOFs may not be the same. Thisimplies it is not possible to predict the structure and/or properties ofone MOFs from another or even from the starting materials used to formMOFs.

In the present disclosure, the expression “composite nanocrystal” isused to refer to MOF nanocrystals and not the resultant metalnanoparticles that have undergone carbonization. The resultant metalnanoparticles that have undergone carbonization are structurallydistinct from the MOF composite nanocrystals that are used in theforming and growing steps. Nevertheless, the resultant metalnanoparticles may be derived from the composite nanocrystals.

In the present disclosure, the composite nanocrystals may comprisezeolitic imidazolate frameworks (ZIFs) crystals. In various embodiments,the composite nanocrystals may comprise zeolitic imidazolateframework-67 (ZIF-67) crystals. ZIF may be considered a class ofmetal-organic frameworks that may be topologically isomorphic withzeolites. This does not mean that the ZIF crystals comprise componentsof zeolites. Instead, the ZIFs may be composed of tetrahedrallycoordinated transition metal ions (e.g. Co) connected by imidazolateligands. ZIFs may have zeolite-like topologies. The ZIF crystals may benanocrystals.

In the forming step of the fibrous support structure, the polymersolution may be formed by dissolving a polymer in an organic solvent at50° C. to 80° C. for 0.5 hours to 2 hours. For example, the dissolutionmay be carried out at 60° C. for 0.5 hours.

The polymer may be selected from the group consisting ofpolyacrylonitrile, phenolic resins, polypyrrole, polystyrene,polymethylacrylonitrile, polyaromatic hydrocarbons, biomass-derivedpolymers and their combination thereof. In various embodiments, thepolymer may comprise or consist of polyacrylonitrile (PAN). PAN may beadvantageously selected because of its mechanical strength to formfree-standing fibrous structure or fibrous support structure or theelectrospun fibers, which may be further converted to carbon fibers. Theexpression “free-standing”, in the context of the present disclosure,means that no templates or other structures are required to support theelectronspun fibers, fibrous support structure or even the resultantfibrous structure.

Meanwhile, an organic solvent may be needed to dissolve the polymer toform the polymer solution. In various embodiments, the organic solventmay comprise or consist dimethylformamide (DMF). The dissolution may becarried out with stirring at temperatures higher than room temperaturesto accelerate dissolution of the polymer in the organic solvent. Thepolymer solution may be cooled to room temperature before further use.

In various embodiments, the weight ratio of the composite nanocrystalsto the polymer may be in the range 2:8 to 6:4, 2:8 to 5:5, 3:7 to 6:4,3:7 to 5:5, 3:6 to 5:5, 3:5 to 5:5, 4:5 to 5:5, 3:7 to 4:5, 3:6 to 4:5,3:5 to 5:5 or any other ratio as specified within these ranges. Withrespect to these ratios, the polymer referred to is the polymer used inthe solution for electrospinning. In various instances, the ratio may be3:7 to 5:5. In various embodiments, the composite nanocrystals and thepolymer may have a weight ratio of 2:8 to 6:4. Advantageously, theweight ratios as disclosed above help to form well-defined electrospunMOF-impregnated fibrous support structure, electrospun fibers or thefibrous membrane with sufficient nucleation sites for growth of the MOFcrystals (i.e. the composite nanocrystals) in the growing step of thepresent method. This step may be called a secondary growth step becausethis step is used to grow the MOF composite nanocrystals formed on thefibrous support structure in the earlier forming step.

Once the polymer solution and the composite nanocrystals (i.e. the MOFcrystals or nanocrystals) are ready, the latter may be dispersed in thepolymer solution to form a suspension. Prior to the dispersion, thecomposite nanocrystals may be dispersed in an organic solvent (e.g. DMF)under sonication for 5 minutes to 30 minutes, e.g. 10 minutes. Thismeans the composite nanocrystals may be in the form of a solution beforecontacting the polymer solution. The dispersion of the MOF compositenanocrystals advantageously avoids aggregation of the MOF compositenanocrystals. The solvent used may also depend on the polymer. In thisinstance, DMF may be suitably used when the polymer solution comprisesPAN. Once in the solution form, the composite nanocrystals may beinjected into the polymer solution. This suspension may then be left tostir overnight.

Once the electrospinning solution is prepared, the electrospinning maybe carried out with an air humidity of not more than 40%, not more than30%, not more than 20%, not more than 10% or not more than 5%. Theelectrospinning may be carried out at a voltage of 7.5 kV to 13 kV, or8.5 kV to 9.5 kV, or any other voltage or voltage range within thespecified ranges. The electrospinning may also be carried out with afeeding rate of 0.5 ml/hour to 3 ml/hour, e.g. 0.5 ml/hour. In someinstances, the electrospinning may be carried out at a voltage of 8.5 kVto 9.5 kV with a feeding rate of 0.5 ml/hour. The term “hour” or “hours”may also be abbreviated as “h”, “hr” or “hrs”, where applicable. Forinstance, when expressing the unit for specific capacity of a battery,the time factor in hours may take the form of “h”, i.e. mA h g⁻¹. Inother instances, durations such as 1 hour, 3 hours may be expressed inthe manner as specified and not in any abbreviated forms.

The electrospun fibers may be collected onto a substrate. The substratemay be any suitable substrate, such as but not limited to, an aluminumsubstrate. The collected fibers may be then left to dry overnight invacuum at 40° C. to 100° C., e.g. 70° C. The electrospun fibers may becollected in the form of a fibrous support structure. In other words,the fibers may be electrospun into a fibrous support structure.

The electronspun fibers or fibrous support structure formed based on theabove described procedures may comprise polymeric fibers. The polymericfibers may comprise polyacrylonitrile, phenolic resins, polypyrrole,polystyrene, polymethylacrylonitrile, polyaromatic hydrocarbons,biomass-derived polymers or their combination thereof. In variousembodiments, the polymeric fibers may comprise or consist ofpolyacrylonitrile for the advantage as mentioned above.

As discussed earlier, the composite nanocrystals (i.e. the MOF crystals)prepared according to the present method, particularly the procedures asdescribed above, may comprise metal ions or clusters of metal ions. Themetal ions may comprise or consist of cobalt, iron or zinc ions. Thecobalt ions may be Co²⁺ or Co³⁺. The iron ions may be Fe²⁺ or Fe³⁺. Thezinc ions may be Zn²⁺.

In the next step of the present method, the composite nanocrystalsseeded on the polymeric fibers may be grown. The composite nanocrystalsmay also be embedded, impregnated or doped in the polymeric fibersforming the fibrous support structure in addition to being partiallyexposed at the surface of the fibers and/or located on the surface ofthe fibers. In the present disclosure, the term “doped” or “dotted” maybe used interchangeably to cover one or both configurations as mentionedabove i.e. the location or position of where the composite nanocrystalsor particles (e.g. the metal nanoparticles) may be located with respectto the fibers (e.g. the polymeric fibers or carbonized fibers). Thisstep may be called a seed-mediated growth stage in the presentdisclosure. The fibrous support structure may be peeled off from thesubstrate e.g. aluminum substrate to use for the growing step.

In the present method, the growing step may be carried out by contactingthe fibrous support structure with an organic solution comprising ametal precursor and an organic linker to form a mixture, and incubatingthe mixture to grow the MOF composite nanocrystals. In other words, thegrowing step serves to grow the MOF composite nanocrystals alreadyformed on the fibrous support structure during the earlier forming step.The size range of the MOF seeds formed before the growing step and theMOF composite nanocrystals grown on the fibers may be 100 nm to 300 nm.

The organic solution may comprise methanol. Advantageously, methanol maybe used in various instances because it may dissolve the reactants andmay be easily removed. Meanwhile, according to various embodiments, themetal precursor may comprise a metal nitrate or metal chloride. Themetal nitrate may be a metal nitrate with or without crystal(s) ofwater. The metal precursor may comprise cobalt nitrate, iron nitrate,zinc nitrate, cobalt chloride, iron chloride or zinc chloride. As anon-limiting example, the metal precursor may comprise cobalt nitratehexahydrate, iron nitrate hexahydrate, zinc nitrate hexahydrate, cobaltchloride, iron chloride or zinc chloride. The metal precursor may alsocomprise a combination of any of the specified or suitable precursors.The metal precursor used for mediated growth may be identical to or maynot need to have the same metal ions of the MOF seed compositenanocrystals.

Independently, the organic linker used may or may not comprise the sameligands used in forming the MOF composite nanocrystals as describedabove. In various instances, the organic linker may comprise animidazole based linker. The imidazole based linker may comprise orconsist of Hmim.

The contacting may be carried out by immersing the fibrous supportstructure into the organic solution for incubation. The incubation mayoccur at 40° C. to 80° C. for 1 hour to 3 hours, e.g. 60° C. for 1 hourto 3 hours. This incubation step allows the composite nanocrystals dopedto grow into larger nanocrystals.

After the mediated growth step, the fibrous support structure may bedried to before carbonization. In the present method, the carbonizationmay be carried out in an inert environment and at a temperature of 600°C. to 1000° C. for 2 hours to 3 hours, e.g. 750° C. for 3 hours. Theinert environment may comprise or consist argon, nitrogen or any othersuitable inert gases. In the context of the present disclosure,carbonization may be carried out to convert an organic material into acarbon-based residue via pyrolysis.

Based on the procedures as described above for the present method, themetal-carbon fibrous structure may comprise carbon and cobalt, carbonand iron, or carbon and zinc. The metal component of the resultantmetal-carbon fibrous structure produced based on the method as disclosedherein may depend on the metal ions present in the MOF compositenanocrystals used in the forming step and/or those of the metalprecursors. For example, if the MOF composite nanocrystals are formedfrom zinc components and the metal precursor is a zinc based precursor,then the resultant metal-carbon fibrous structure is a zinc-carbonfibrous structure. In another example, if the MOF composite nanocrystalsare formed from cobalt components and the metal precursor is an ironbased precursor, then the resultant metal-carbon fibrous structure maycomprise cobalt and iron as the metal component.

The present method may further comprise a step of calcinating themetal-carbon fibrous structure to yield a metal oxide-carbon fibrousstructure. The calcinating may be carried out at 200° C. to 350° C.,e.g. 300° C. for 1 hour. The calcinating step may be carried out in air.

The calcination may be used to convert the metal component (e.g. themetal nanoparticles) of the carbonized fibrous structure to a metaloxide so that it may be used as an anode material, for instance, inlithium ion batteries. The metal oxide-carbon fibrous structure maycomprise Co₃O₄ and carbon, ZnO and carbon, Fe₂O₃ and/or Fe₃O₄ andcarbon.

As can be seen from the above procedures described, the present methodmay be carried out without using any templates. This means that noadditional support structures may be used to form the resultantfree-standing metal-carbon fibrous structure other than the use of theelectrospun fibers itself.

In the present disclosure, there may also be a metal-carbon fibrousstructure comprising carbon based fibers arranged to form a porousnetwork and the carbon based fibers are doped with metal nanoparticles,wherein the carbon based fibers have surfaces which comprise graphiticcarbon. The metal-carbon fibrous structure may be in the form of afibrous mat or membrane.

In various embodiments, the carbon based fibers may comprise hollowporous carbon nanocages arranged along a single dimension to form thecarbon based fibers. The carbon based fibers may comprise a diameter inthe range of 300 nm to 1000 nm, 300 nm to 550 nm, 550 nm to 1000 nm, 450nm to 550 nm, 500 nm to 550 nm, 450 nm to 500 nm or any other diameteror ranges as specified within these ranges. The diameter may be taken asthe cross-section of the fiber. The carbon nanocages or walls of thecarbon nanocages may be doped with the metal nanoparticles. The walls ofthe carbon nanocages may be used to form the walls of the fibers makingup the fibrous structure.

In various embodiments, the resultant metal nanoparticles doped on thefibers of the fibrous structure may have a size of 5 nm to 50 nm orother size within this specified range. For example, the resultant metalnanoparticles may be around 10 nm. The metal nanoparticles may comprisecobalt, iron or zinc nanoparticles. The iron may be iron (II) or iron(III).

As disclosed in the method embodiments, the size of the MOF seeds andMOF composite nanocrystals grown on the fibers may be similar in therange of 100 nm to 300 nm or 100 nm to 200 nm. The difference in sizebetween the MOF composite nanocrystals and the resultant carbonizedmetal nanoparticles may be because carbonization is performed at atemperature much higher than the decomposition temperature of the MOFcomposite nanocrystals. Further, all organic components would have beendecomposed by carbonization. The MOF composite nanocrystals may alsohave been converted to nanocages doped with metal nanoparticles.

The metal content of the metal-carbon fibrous structure may be up to 50wt %, up to 40 wt %, up to 30 wt %, up to 20 wt % etc. For example, themetal content may be up to 24.6 wt %. The wt % may be based on theweight of the resultant metal-carbon fibrous structure.

In various embodiments, the metal-carbon fibrous structure may befurther calcined to form a metal oxide-carbon fibrous structure. Themetal oxide-carbon fibrous structure may comprise Co₃O₄ and carbon, ZnOand carbon, Fe₂O₃ and/or Fe₃O₄ and carbon. In the present disclosure,the resultant fibrous structure before calcination may also be calledthe carbon fibrous structure because the polymeric fibers is likely tobe thermally converted to carbon after carbonization.

In summary, the present disclosure relates to a method of preparingfree-standing hybrid porous carbon nanofibrous mats based on MOFmaterials via electrospinning, seed-mediated secondary growth followedby thermal treatment. The said hybrid porous carbon nanofibers exhibit aunique structure which may be in the form of a 1D assembly ofinterconnected MOF derived hollow carbon nanocages with high porevolume, hierarchical pore size, well-graphitized carbon wall and highcontent of metal nanoparticles uniformly dispersed in the carbon matrix.

In the present disclosure, free-standing porous hybrid carbonnanofibrous mats may be first fabricated via direct carbonization of MOFbased electrospun fibers. In the preparation of the MOF basedelectrospun fibrous mats, nanosized (100 nm to 200 nm) MOF crystals maybe grown on the support fibers using the seed-mediated procedure asdisclosed herein under a mild condition (e.g. 60° C.) and in a shortperiod of time (1 hour to 3 hours). The porous carbon nanofibers mayexhibit a unique structure as disclosed above i.e. the 1D assembly ofinterconnected hollow carbon nanocages derived from MOF crystals. Theporous carbon fibrous mats may possess high pore volume and hierarchicalpore size which may be fabricated without using any “hard” or “soft”templates. Due to the ordered structure of MOF crystals, the derivedporous carbon nanofibers may possess high proportion of graphitizedcarbon and rich content of fine metal particles (about 10 nm) uniformlydispersed in the carbon matrix.

While the methods described above are illustrated and described as aseries of steps or events, it will be appreciated that any ordering ofsuch steps or events are not to be interpreted in a limiting sense. Forexample, some steps may occur in different orders and/or concurrentlywith other steps or events apart from those illustrated and/or describedherein. In addition, not all illustrated steps may be required toimplement one or more aspects or embodiments described herein. Also, oneor more of the steps depicted herein may be carried out in one or moreseparate acts and/or phases.

EXAMPLES

The present disclosure relates to the field of nanostructured hybridporous carbon materials. Specifically, the present disclosure provides ageneral and scalable method for preparing MOF-derived free-standingporous carbon nanofibrous mats dotted with fine metal particles via thesteps as disclosed above i.e. electrospinning, seed-mediated secondarygrowth followed by thermal treatment. The resultant hybrid carbonnanofibers may exhibit a unique structure which comprises the onedimensional (1D) assembly of interconnected MOF derived hollow carbonnanocages with high pore volume, hierarchical pore size,well-graphitized carbon wall and high content of metal nanoparticlesuniformly dispersed in the carbon matrix. As disclosed herein, thematerials can be assessed as anode materials for lithium ion-batteries,where they have exhibited excellent performance with specific capacitiesup to 1074 mA h g⁻¹, good cyclic stability and high rate performance. Byusing different MOF crystals based on this methodology, these materialsmay possess great potential for wide applications in energy storageand/or conversion, sensing, gas scavenging, catalysis, water treatmentetc.

While the invention has been particularly shown and described withreference to specific embodiments, it should be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. The scope of the invention is thusindicated by the appended claims and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced.

Example 1: Preparation of ZIF-67 Nanocrystals

900 mg of cobalt nitrate hexahydrate was dissolved in 3 ml deionized(DI) water. 11000 mg of 2-methylimidazole (Hmim) was then dissolved in20 ml DI H₂O under sonication. These two solutions were mixed andstirred for 1 hour at room temperature. The resulting purpleprecipitates were collected by centrifuging, washing with water and thenmethanol for 3 times, and finally vacuum dried at 80° C. for 24 hours.

Example 2: Preparation of ZIF-67 Embedded Electrospun Fibers

328 mg polyacrylonitrile (PAN, Mw is 150000) was dissolved in 2.0 g ofN,N-dimethylformamide (DMF) under magnetic stirring at 60° C. for 0.5hours and then cooled down to room temperature. 218 mg of the ZIF-67nanocrystals (100 nm to 200 nm) prepared in example 1 were dispersed in1.77 g of DMF under sonication for 10 minutes. The suspension was theninjected into the above PAN solution under stirring. After continuousstirring overnight, the mixture was placed in a 2 ml plastic syringefitted with a flap tip 22 G needle and was electrospun using ahorizontal electrospinning setup with air humidity lower than 30%.Electrospinning was performed at 8.5 kV to 9.5 kV with a feeding rate of0.5 ml/hour and the needle tip-to-plate substrate distance was 10 cm.The nanofibers were collected on aluminum foil and dried at 70° C. undervacuum overnight.

Example 3: Growth of ZIF-67 Nanocrystals on The ZIF-67/PAN ElectrospunFibers

The electrospun fibrous mats of example 2 were peeled off the aluminumfilm and immersed in a solution of 17.6 mg cobalt chloride dissolved in3 ml methanol. Another 89.5 mg of Hmim was separately dissolved in 3 mlof methanol and added to the former mixture slowly to have uniformgrowth. Immediate precipitation can be observed based on colour changeof pink to dark blue solution. The resultant mixture was then kept inthe oven at 60° C. for 1 hour. Upon cooling, the dark blue fibers werewashed thoroughly with cold methanol several times and finally dried inthe oven at 80° C. for 24 hours.

Example 4: Preparation of the ZIF-67 Derived Hybrid Carbon Fibers

The dried nanofibrous mats were directly carbonized in tube furnaceunder a flow of argon (150 cm³ STP/min). The temperature was ramped from25° C. to 750° C. at 5° C. min⁻¹ and kept at 750° C. for 3 hours toyield the cobalt/carbon nanofibers based on ZIF-67. This was furtherconverted to Co₃O₄/carbon nanofibers via calcination at 300° C. for 1hour.

Example 5: Characterization

Morphology of the MOF nanocrystals and hybrid nanofibers were observedunder JEOL JSM 6700 field emission scanning electron microscope at anaccelerating voltage of 5 kV. All samples were coated with a thin goldlayer before SEM imaging.

TEM images were obtained with Philips CM300 FEGTEM high resolutiontransmission electron microscope.

Wide-angle X-ray diffraction (XRD) measurements were performed using aBruker D8 Discover GADDS X-ray diffraction meter with Cu Kα radiationand Raman spectra were recorded on Jobin Yvon T64000 triple spectrographmicro-Raman system.

The metal content in the resultant hybrid carbon fibers was measured byinductively coupled plasma mass spectrometry (ICP-MS) analysis.

The specific surface area and pore size analysis was measured using aMicromeritics ASAP 2020 system.

Example 6: Electrochemical Performance

Electrochemical performance of the MOF based carbon fibers wereevaluated as anode in lithium ion batteries. The free-standing hybridcarbon fibrous mats were punched into discs of 15 mm in diameter andassembled into 2032 button cells in an argon filled glove box withlithium foil, Celgard2325 membrane and 1 M LiPF₆ in ethylenecarbonate/dimethylcarbonate (1:1 v/v ratio), copper/aluminum foil as thecounter electrode, separator, electrolyte and current collectors,respectively. The charge-discharge testing was conducted on NEWAREbattery tester at different current densities with a cutoff voltagewindow of 0.005 V to 3.0 V. Rate capacities were obtained viadischarging at 50 mA g⁻¹ and charging at various current densities.

Example 7: An Exemplary Non-Limiting Embodiment of the Present Method

An exemplary non-limiting embodiment of the present method as describedabove and in conjunction with the figures, particularly FIG. 1 a to FIG.1 g , follows below.

FIG. 1 a to FIG. 1 g depict a scalable method as described herein forpreparing free-standing MOF derived hybrid porous carbon nanofibrousmats dotted (i.e. doped) with fine metal particles via electrospinning,seed-mediated secondary growth followed by thermal treatment.

Fiber formation 104 may be first carried out via an electrospinningsetup as shown in FIG. 1 a . The electrospinning setup has a highvoltage source 100 and a syringe where the voltage is applied inproximity of a needle of the syringe. The syringe contains a polymersolution 102 with MOF nanocrystals. The fibers are then formed as a MOFembedded electrospun fibrous mat as shown in FIG. 1 b . The mat is thensoaked in a solution of the metal salt precursor followed by slowlyadding the organic linker solution as shown in both left and rightimages of FIG. 1 c , respectively. The MOF nanocrystals are then allowedto undergo secondary growth under mild conditions (60° C.) with a shortduration (e.g. 1 hour to 3 hours) as illustrated in FIG. 1 d . The matis then washed repeatedly with solvent in FIG. 1 e.

The MOF based fibrous mat is then retrieved (FIG. 10 , dried and furthercarbonized to become the MOF based carbon fibrous mat in FIG. 1 g.

Example 8: Summarized Details and Results of the Present Method

As shown in FIG. 1 a to FIG. 1 g , the disclosed MOF derived carbonfibrous mats were fabricated via a three step process which firstinvolves the preparation of MOF embedded electrospun polymer fibers.Continuous seed-mediated growth of MOF nanocrystals on the fibers isthen carried out and followed by direct carbonization with or withoutfurther heat treatment to yield free-standing hybrid carbon fibrous matswell dispersed with nano-sized metal or metal oxide particles.

One of the polymers used for electrospinning is polyacrylonitrile (PAN)as it generates free-standing nanofibrous mats with good mechanicalstrength and is commonly used for fabrication of carbon fibers. Otherpolymers may include, but not limited to, phenolic resins, polypyrrole,polystyrene, polymethylacrylonitrile, polyaromatic hydrocarbons,biomass-derived polymers and the combination thereof.

The MOF nanocrystals used in the method as disclosed herein areaccording to example 1. The MOF nanocrystals produced are with sizesaround 100 nm to 200 nm and the weight ratio of MOF to polymer can betuned from 3:7 to 5:5 to ensure a well-defined electrospun MOFimpregnated fibrous membrane with sufficient nucleation sites for thesecondary growth of MOF crystals (see example 2).

The hybrid fibrous mats were then immersed in the solution of metalprecursor mixed with organic linkers at elevated temperature for 1 hourto 3 hours to continuously grow MOF nanocrystals on the fibers (seeexample 3).

The integrated MOF based electrospun fibers were subsequently subjectedto direct carbonization under argon above 600° C. to generatefree-standing hybrid carbon fibrous mats incorporated with metals, whichcould be further converted to metal oxides via post heat treatment inthe air (see example 4).

FIG. 2 a and FIG. 2 b present SEM images of the prepared ZIF-67nanocrystals with sizes around 200 nm and the corresponding electrospunZIF-67/PAN fibers. The diameters of the hybrid fibers could becontrolled at around 500 nm via adjusting the spinning parameters toimpregnate all of the MOF nanocrystals in the fibers, which providenucleation sites to facilitate the continuous growth of MOF crystals.

As shown in FIG. 3 a to FIG. 3 c , after immersion in the solution ofcobalt precursor and organic linker for one hour at 60° C., the hybridelectrospun fibers are almost completely covered with ZIF-67nanocrystals of sizes ranging from 150 nm to 200 nm. The XRD patterns ofpure ZIF-67 nanocrystals, as-spun ZIF-67/PAN fibers and ZIF-67/PANfibers after secondary growth are shown in FIG. 4 , which all exhibitthe characteristic peaks of the standard ZIF-67 crystals and alsoclearly demonstrate that the structure of the ZIF-67 nanocrystals remainintact during the electrospinning process and are partially exposed onthe fiber surface to act as seeds for the secondary growth.

As shown in FIG. 5 , after direct carbonization at 750° C., the ZIF-67based electrospun fibers can still maintain the free-standingnanofibrous morphology to yield the hybrid cobalt/carbon fibers, asevidenced by their X-ray diffraction (XRD) patterns (FIG. 9 ). Asrevealed in the TEM images shown in FIG. 6 , these carbon fibers exhibita porous network structure composed of the MOF derived nano-sized hollowcarbon nanocages as building blocks in one dimension (1D). From the highresolution TEM images shown in FIG. 7 , the carbon walls are wellgraphitized and uniformly doped with very fine cobalt nanoparticles withsizes around 10 nm. The content of the metal can reach as high as 24.6wt % by inductively coupled plasma mass spectrometry (ICP-MS) elementalanalysis. The Raman spectrum of the ZIF-67 derived carbon fibers isshown in FIG. 8 . The curve that is outlined according to the spectrumrepresents the sum of the two components responsible for the D and Gpeaks. The peak intensity ratio of the D band (1350 cm⁻¹, disorderedcarbon) to the G band (1580 cm⁻¹, sp² carbon) is as low as 0.73. Thisindicates a high proportion of graphitic carbon was formed under thecatalysis of cobalt and also due to the ordered crystal structure of theMOF nanoparticles. After post calcination at 300° C. for 1 hour, theporous cobalt/carbon nanofibers was successfully converted toCo₃O₄/carbon nanofibers, which is promising for application as anodematerial for lithium ion batteries.

FIG. 10 and FIG. 11 show the N₂ adsorption-desorption isotherms and thecorresponding BJH pore size distribution curve for the ZIF-67 derivedcarbon nanofibers, respectively. The material exhibits a type-IV curvewith a dramatic hysteresis loop in the range of P/P₀ to be 0.4 to 1,which is a characteristic of hollow mesoporous materials.

According to the Barrett-Joyer-Halenda (BJH) model, the ZIF-67 derivedcarbon nanofibers possess plenty of mesopores with a maximum diameterfrequency centered at or near 1 nm, 3 nm and 40 nm. The specific surfacearea estimated by the Brunauer-Emmett-Teller (BET) method is 257 m²/gwith a high total pore volume of 0.485 cm³/g, in which the microporesaccounts for 11% with a pore volume of 0.052 cm³/g. The above BET reportdemonstrates that this kind of nanoporous carbon possesses richhierarchical micro-mesopores that may bring about plenty ofaccommodation sites for Lithium ion storage and also provide optimalpathways to facilitate mass transportation, thereby ensuring the highperformance in energy storage systems.

The electrochemical performance of the ZIF-67 derived porousCo₃O₄/carbon fibers was investigated as the anode in Lithium ionbatteries. FIG. 12 shows the initial charge/discharge curves of thematerial at a low current density of 50 mA g⁻¹ for the first ten cycles.The material exhibited a high charge capacity (high initial specificcapacity) up to 1074 mA h g⁻¹ and excellent cyclic stability (FIG. 13 ).The coulombic efficiency increased from 65.7% to 97.3% in the first tencycles. Remarkably, this material also shows good performance at highcurrent densities. As shown in FIG. 14 , the specific capacity decreasesvery slowly when increasing the charging currents step by step. Evenwhen the current density is increased from 50 mA g⁻¹ to 5 A g⁻¹, aspecific capacity of 320 mA h g⁻¹ is still retained. The rate capacityalmost rebounds back to the original value and keeps stable when a lowrate of 100 mA g⁻¹ was applied again. A high specific capacity of over800 mA h g⁻¹ is attainable at a current density of 1 A g⁻¹. The goodhigh rate performance of the carbon spheres can be attributed to thegraphitic layers for improving electrical conductivity as well as thequantities of interconnected mesopores which provide short pathways forlithium ion diffusion and electron transportation.

As shown in FIG. 15 , the material displays excellent cyclic stabilitywhen evaluated at a current density of 100 mA g⁻¹ up to 50 cycles,highlighting the great potential of this material for practicalapplications.

Example 9: Applications and Utilities

Unlike conventional methods that fabricate MOF derived carbon materialsas bulk porous particles, the present method as disclosed herein firstobtains free-standing, interconnected carbon architectures derived fromMOF assemblies, which have improved structural integrity and conveniencein handling and recycling for specific applications.

With inexpensive MOF crystals as building blocks, the present methodfabricates porous nanocarbon materials via a straightfoward,template-free and cost effective process, which provides a methodologythat can be applied to MOF crystals based on various metals fordifferent applications.

The obtained hybrid porous carbon fibrous mats as disclosed herein havebeen shown to outperform conventional porous carbon fibrous structurefor energy storage. For instance, the present fibrous structure has highreversible specific capacities, good high rate performance and excellentcyclic stability when applied as anode in lithium ion batteries.

Accordingly, the method of the present disclosure can be used tofabricate free-standing hybrid porous carbon nanofibrous mats for a widerange of applications, not only as electrodes in energy storage orconversion systems, but also as substrates for sensing, gas scavenging,catalysis, air and/or water treatment, toxic substance management etc.

The invention claimed is:
 1. A metal-carbon fibrous structure comprisingcarbon based fibers arranged to form a porous network, wherein thecarbon based fibers have surfaces which comprise graphitic carbon,wherein the carbon based fibers comprise hollow porous carbon nanocageshaving walls which define the surfaces of the carbon based fibers,wherein the hollow porous carbon nanocages are doped with metalnanoparticles, and wherein the metal nanoparticles are embedded inand/or partially exposed on the surfaces of the carbon based fibers. 2.The metal-carbon fibrous structure according to claim 1, wherein thehollow porous carbon nanocages are arranged along a single dimension toform the carbon based fibers.
 3. The metal-carbon fibrous structureaccording to claim 1, wherein the carbon based fibers comprise adiameter in the range of 300 nm to 1000 nm.
 4. The metal-carbon fibrousstructure according to claim 1, wherein the metal nanoparticles have asize of 5 nm to 50 nm.
 5. The metal-carbon fibrous structure accordingto claim 1, wherein the metal nanoparticles comprise cobalt, iron orzinc nanoparticles.
 6. The metal-carbon fibrous structure according toclaim 1, wherein the metal-carbon fibrous structure comprises a metalcontent of up to 50 wt %, wherein the wt % is based on the weight ofmetal-carbon fibrous structure.
 7. The metal-carbon fibrous structureaccording to claim 1, wherein the metal-carbon fibrous structure isfurther calcined to form a metal oxide-carbon fibrous structure.
 8. Themetal-carbon fibrous structure according to claim 7, wherein the metaloxide-carbon fibrous structure comprises Co₃O₄ and carbon, ZnO andcarbon, Fe₂O₃ and/or Fe₃O₄ and carbon.